bei48482_FM

road to a Theory of Everything. But thus far its predictions cannot be directly con-

fronted with the results of experiment, so there is no way to know whether minute

loops of string in ten dimensions actually exist with their vibrations making up the

world we see around us.

13.8 HISTORY OF THE UNIVERSE

It began with a bang

The observed uniform expansion of the universe points to a Big Bang around 13 bil-

lion years ago that started from a singularity in spacetime, a point whose energy den-

sity and spacetime curvature were both infinite. In the absence of a quantum-mechanical

theory of gravity, nothing can be said about the immediate aftermath of the Big Bang.

After 10^43 s, however, the theory that ties together the strong, electromagnetic, and

weak interactions, even though incomplete, permits a general picture to be sketched

of what may well have happened.

As the initial compact, intensely hot fireball of matter and radiation from the Big

Bang expanded, it cooled and underwent a series of transitions at specific tempera-

tures. An analogy is with the cooling of steam, which becomes water and then ice as

its temperature falls. Figure 13.12 shows the different phases of the universe on a graph

of temperature (actually kT) versus time, both on logarithmic scales. The unit of kT

here is the electronvolt, where 10^4 eV corresponds to1 K.

498 Chapter Thirteen

Time since the Big Bang, s

Strong interaction frozen

Quark-lepton era

The present

Quantum gravity

Unified era

Hadron-lepton era

Nuclei form

Nuclei-electron

era

Atomicera

10 –^4010 –^3010 –^2010 –^10110101020

1028

1024

1020

1016

1012

108

104

1

10 –^4

Electromagnetic interaction

frozen out

Galaxies and stars

begin to form

Quarks condense into hadrons

Radiation

dominant

Matter

dominant

Temperature, eV Weak interaction frozen out

Figure 13.12Thermal history of the universe on the basis of current theories. Nothing can be said about

the state of the universe until 10^43 s after the Big Bang in the absence of a quantum-mechanical theory

of gravity.

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